The present invention relates to a receiver integrated circuit (hereinafter refereed to as a receiver IC) for use with a mobile telephone and, more particularly, to a receiver IC for a mobile telephone operating in a dual mode environment involving CDMA (code division multiple access) mode and FM (frequency modulation) mode.
FIG. 4 is a block diagram of a common mobile telephone operating in a dual mode environment involving CDMA mode and FM mode. The transmitter (TX) portion of the telephone works and is constituted as follows: signals I and Q output by a modem 101 are QPSK-modulated by a quadri-phase shift keying (QPSK) modulator 102. The modulated signals are amplified by a transmitter-side variable gain amplifier (TX-AMP; simply called a variable amplifier hereunder) 103. The amplified signals are mixed by a mixer (MIX) 104 together with a locally oscillated signal from a local oscillator (OSC) 121, whereby a radio frequency (RF) transmitted signal is produced. The RF transmitted signal is transmitted via a band-pass filter 105, a power amplifier (PA) 106, a duplexer 107 and an antenna 108.
The receiver (RX) portion of the mobile telephone works and is constituted as follows: an RF signal received via the antenna 108 is sent to a mixer (MIX) 111 by way of the duplexer 107, a low-noise amplifier (LNA) 109 and a band-pass filter 110. The received signal is mixed by the mixer 111 together with the locally oscillated signal from the local oscillator (OSC) 121, whereby an IF received signal is produced. The IF received signal is applied both to a CDMA band-pass filter 112 and to an FM band-pass filter 113. One of output signals from the two filters is selected in accordance with a currently established mode. The selected output signal from the filter is amplified by a receiver-side variable amplifier (RX-AMP) 114. The amplified signal is demodulated by a QPSK demodulator 115, and the demodulated signal is sent to the modem 101.
In the modem 101, a received signal intensity indication circuit 116 detects the intensity of the received signal. The detected reception intensity is compared with intensity reference data by a comparator 117. The difference between the detected intensity and the reference data is sent both to a receiver-side AGC voltage correction circuit 118 and to a transmission output correction circuit 119. The receiver-side AGC voltage correction circuit outputs an AGC voltage such that the difference sent from the comparator 117 will become zero, i.e., that the output of the circuit 116 will coincide with the reference intensity data, whereby the gain of the receiver-side variable amplifier (RX-AMP) 114 is controlled.
The transmission output correction circuit 119 on the transmitter side is supplied both with the difference from the comparator 117 and with transmission output correction data reflecting the line status between the mobile telephone and a base station. A transmitter-side AGC voltage correction circuit 120 outputs an AGC voltage according to the transmission output correction data to control the gain of the variable amplifier (TX-AMP) 103 so that the modulated signal will be inversely proportional to the level of the received signal.
When the mobile telephone of the above constitution is in CDMA mode, signals of -105 dBm through -25 dBm are input to the antenna 108. This means that the receiver portion must have a dynamic range of at least 80 dB for the received signals. In FM mode, signals of -120 dBm through -20 dBm enter the antenna 108. In the latter case, the receiver portion must have a dynamic range of at least 100 dB.
Such extensive dynamic ranges are implemented by the receiver-side variable amplifier 114 having a plurality of variable amplifiers cascaded as shown in FIG. 5, so that an IF received signal with pronounced level fluctuations will have a constant level at an input terminal of the QPSK demodulator 115. Harmonic components of the output signal from the variable amplifier 114 are attenuated by the low-pass filter (LPF) 4, and the resulting signal is input to the QPSK demodulator 115. By resorting to a 90-degree phase shifter 8, a PLL divider (1/N) 9 and an oscillator 10, the QPSK demodulator 115 demodulates the input signal back to the initial base-band signals I and Q.
In CDMA mode, signals of -105 dBm to -25 dBm entering the antenna are amplified by the variable amplifier 114 so that they will have a constant level at the input terminal of the QPSK demodulator 115. In FM mode, of the signals of -120 dBm to -20 dBm entering the antenna, those of -120 dBm to -40 dBm (i.e., for 80 dB) are amplified by the variable amplifier 114. The remaining signals of -40 dBm to -20 dBm, exceeding the control range of the variable amplifier 114, are saturated therein and forwarded at a constant level to the QPSK demodulator 115 through limiter operation.
The signals saturated in the variable amplifier 114 generate harmonic components twice to four times those of their unsaturated counterparts. These harmonic components are attenuated by the LPF 4 to 20 dB or less with respect to a fundamental wave. The characteristic of the LPF 4 is illustrated in FIG. 2. The output signal of the LPF 4 has a CN ratio greater than 20 dB. The QPSK demodulator 115 and an FM digital demodulator installed immediately downstream need to provide a CN ratio greater than 20 dB where the SN ratio of the demodulated signal is desired to be greater than 45 dB. The frequency of the oscillator shown in FIG. 5 is set to a frequency input to the variable amplifier 114.
In the conventional setup outlined above, harmonics, especially those of degree two, are characterized for their frequency being close to the signal and having high voltages. This requires that the LPF 4 for attenuating harmonics provide a steep cut-off characteristic depicted in FIG. 2. The LPF 4 is thus composed generally of an LC circuit and is not conducive to being implemented in IC form. A resonance coil L, included in the oscillator 10 for QPSK demodulation, is too bulky to be formed in IC. Because the resonance coil L needs adjustment after assembly, it cannot be incorporated in an IC arrangement.
One proposed solution to the above difficulties is a setup in which the LPF 4 (with connecting points 6 and 7) and resonance coil L in FIG. 5 are both separated and externally furnished while the variable amplifier 114, QPSK demodulator 115, 90-degree phase shifter 8, PLL divider 9 and oscillator 10 are built altogether into IC. One problem with this constitution is that the connecting points 6 and 7 of the LPF 4 are poorly isolated from the input terminal of the variable amplifier 114. The inadequate isolation makes it difficult to achieve gain control characteristics of 80 dB or more and tends to deteriorate gain slope linearity.
Experiments have shown that between 100 and 300 MHz, the isolation between terminals is about 40 to 50 dB. If the amplifier gain is set to -50 to+40 dB to satisfy other requirements, the necessary attenuation of -50 dB cannot be accomplished. In addition, where lead terminals for connecting the LPF 4 are installed in the IC, an oscillated signal from the resonance coil L reaches the connecting points 6 and 7 of the LPF 4 and the input terminal of the amplifier 114. The oscillated signal and a signal derived from limiter operation produces beat in the QPSK demodulator 115, whereby the CN ratio of the signals I and Q is deteriorated.